1. Field of the Invention
This invention pertains to the completion of oil and gas wells, and more particularly, to a method and apparatus for running a string of casing and casing hanger into the well, removal of the handling string and run-in tool, and tying back the casing hanger with the surface by means of a tie back tool and insulated riser casing. The apparatus includes a particular casing hanger adapted for cooperation with special run-in and tie back tools. The invention is particularly directed to running in a string of casing with a handling string, cementing the casing by circulating fluid through the handling string and back up around the annulus, removing the handling string, and tying back into the casing hanger with an insulated riser for completing a well through permafrost.
2. Description of the Prior Art
In the completion of oil and gas wells, often it becomes desirable to suspend a string of casing from a location within the well which is not only below but spaced from the level of the Christmas tree. This may occur, for example, where the well is under water or extends through permafrost. Initially, in the drilling of such a well an outer conductor casing is installed in the ground with a well head at or below the level of the bottom of the water or permafrost and a conductor riser extending to the top of the water or permafrost. Then a varying number of strings of casing and tubing are suspended within the conductor casing. Each of these latter strings of pipe is attached to a hanger which is supported by a head affixed to the previously installed casing. When the casing is to be suspended a hanger is attached to the upper end of the string and the hanger is attached to a handling tool. The handling tool is affixed to a handling string, generally made up of drilling casing, which is used to run the hanger into the well through the conductor riser to the level of the well head in which the hanger is to be suspended. The handling tool may later be disconnected from the hanger and the tool and handling string removed from the well. To one of the casing hangers may then be connected a drilling riser extending up to the top of the water or permafrost where the Christmas tree is to be installed. This is achieved by affixing a tie back extension tool to the drilling riser and running the assembly into the well for tying back with the casing hanger.
Although there are various reasons and occasions for suspending casing within the well, a level spaced below the Christmas tree, the two most common occur while drilling offshore or while drilling in a permafrost region. Permafrost is permanently frozen mud which is found, for example, on the north slope of Alaska. Although the present invention is applicable to any situation requiring the suspension of casing in remote location, the present invention was primarily designed for problems occurring in a well drilled through permafrost. In a permafrost region the casing must be suspended in solid ground or rock below the interface (permafrost line) of the permafrost and solid ground because of the possible subsidence characteristics of the permafrost.
The present invention concerns the problem of the heat given off by the oil as it is produced from a well which has been drilled through permafrost. As the oil rises it will give off heat which will melt the permafrost if standard casing has been used. This turns the permafrost into mud around the outside of the conductor casing causing subsidence which is undesirable. Such subsidence causes the ground supporting the base of the drilling and production platform to shift and sink. Therefore, insulation is used to insulate the permafrost from the heat given off by the oil. This insulation is generally achieved by using an insulated production casing which extends from the hanger to the surface thereby insulating the oil as it flows through the region surrounded by permafrost.
Problems are created in the completion of such a well since insulated production casing cannot be used to lower the hanger into the well. Standard riser casing must be used since it has a smaller outside diameter. The smaller diameter is required because upon the installation of the hanger the seals are tested and the production tubing is perforated for the production of oil or gas. If insulated riser casing were used to lower the hanger, the added diameter of the riser casing would prevent the seal tests and perforation since the circulation of any drilling fluid through the annulus between the insulated riser casing and the conductor casing would be inhibited. Also, the casing string is cemented after the hanger has been installed thereby requiring the flow of cement down through the handling string and requiring the returns from the cement to pass through the annulus. Standard drilling riser casing also permits a greater degree of control during this portion of the completion of the well. Standard drilling riser casing cannot be used during the production of the well due to its being uninsulated. Therefore, because of these difficulties, the hanger is lowered using standard drilling riser casing and then later replaced with insulated production riser casing prior to production. The drilling riser is generally replaced with a new production riser anyway because the drilling riser often becomes worn and damaged during the drilling operation thereby not being capable of withstanding the greater pressures in the well during the production operation.
Completions of wells drilled through permafrost are few in number and, therefore, little prior art is available on apparatus which may have been used to overcome these difficulties. Presently, it is presumed that the casing hanger is merely lowered by means of a conventional threaded connection between the handling tool and casing hanger, and the tie-back is later achieved in the same manner by using a conventional threaded connection between the tie-back extension tool and the same threaded portion of the casing hanger as was used for the run-in tool.
Problems arise in using a conventional threaded connection because the connections between the drilling casing making up the handling string are of the same hand as the connection between the handling tool and casing hanger. Therefore, when the handling string is rotated to disconnect the handling tool from the hanger, the connections between the sections of pipe making up the handling string also unthread. These problems have been solved in connection with noninsulated risers used as the production casing string by using left hand threaded connections between the risers and hangers. These are called back off joints. Such joints are suitable, where the handling string on which the casing is run is suitable for use as a riser and need not be removed unless the well is to be abandoned, or where the particular casing is not needed for production and need not be replaced after removal. The present invention solves the problem occurring when it is necessary to tie in with insulated riser casing after removal of the handling string. Examples of the use of left hand thread back off joints are to be found in U.S. patent application Ser. Nos. 103,839 and 76,664.
Several other problems occurred when operating off of two threads. The casing would get stuck in the connection and it could not be either tightened or loosened from the connection. This would cause galling of the threads, upon tying back with insulated riser casing, a seal could not be re-established in the connection and often so much torque was lost that there would not be sufficient torque at the wellhead to make the connection.
The use of the conventional threaded connection between the casing hanger and the tie-back extension tool also causes extreme wear on the seals which have been compressed between the inner surface of the casing hanger and the outer surface of the tie-back extension tool. The tie-back extension tool is necessarily rotated to complete the threaded connection with the casing hanger and this causes frictional wear on the seals as they rub between the two surfaces.
The problem of frictional wear on the seal due to rotation with a threaded connection has previously been attacked in connection with casing suspension apparatus as shown by U.S. Pat. No. 3,588,130-Fowler. See also U.S. Pat. No. 3,540,533-Morrill. The use of a rotatable connection between two well pipe elements is also shown in U.S. patent application Ser. No. 87,783. A running tool for well casing is disclosed in U.S. Pat. No. 3,489,436 (issued Jan. 13, 1971 on the application of A. G. Ahlstone).
Overcoming the problem of making and breaking connection with a remotely located well element has been the object of a number of patents, e.g. U.S. Pat. No. 3,330,341 Jackson and Rhodes.